Generally, rise in temperature of a pneumatic tire is not favorable in view of the durability because the temperature rise accelerates over-time changes such as the changes in the properties of a material for the pneumatic tire or causes a break of a tread thereof during a high speed run. Lowering the temperature of the tire has been a significant desideratum to improve the durability especially for a run-flat tire in a case where a passenger vehicle with the run-flat tire is running while the run-flat tire is punctured (running with a 0-kPa internal pressure). For example, if a run-flat tire that has a crescent-shaped reinforcement rubber is running while being punctured, deformation in radial direction concentrates on the reinforcement rubber. This raises the temperature of this portion to a very high temperature, thereby affecting significantly the durability.
An example of means for lowering tire temperature to address the above problem is that in which heat generation is suppressed by use of a reinforcement member designed to reduce or suppress the distortion of tire components (e.g., carcass plies). In this case, however, the use of the reinforcement member may exert adverse effects on the ordinary performances of the tire. Such adverse effects include occurrence of unpredictable failures, and deterioration in ride comfort especially when the reinforcement member is used in a run-flat tire. The deterioration in ride comfort of the ran-flat tire is due to higher vertical spring (bouncing in the vertical direction) during a run with tires of normal internal pressures. What is needed accordingly is new means for lowering tire temperature without sacrificing normal performances of the tire.
In an example of such new means for lowering tire temperature (see WO 2007/032405), the cooling effect is improved by forming turbulence generation ridges extending in the radial direction of the tire in tire side portions and by causing the turbulence generation ridges to generate or accelerate turbulence of higher flow speed on the surface of the tire. Rubber, which tires are made of, is a material of poor heat conductivity. So, it is a known fact that the cooling effect by accelerating the turbulence generation is more effective than the cooling effect by expanding the heat-dissipating area.
It turns out, however, that if the turbulence generation ridges are arranged in high density to cover the entire surface of the tire side portions, the tire becomes heavier and has an increased travelling resistance due to a somewhat larger resistance of air caused by the turbulence generation ridges.
Accordingly, an object of the invention is providing a pneumatic tire capable of preventing both the tire weight and the travelling resistance from increasing while maintaining the effect to lower the temperature of tire side portions to a high level.